LOW COST DISTRIBUTED BASE ISOLATION FOR BRICK MASONRY BUILDINGS

Abstract

Earthquake protection of buildings via base isolation has received considerable attention in recent times. A feasible economic isolation solution for low cost masonry buildings lies with base isolation by using pure friction (P-F) interface. The basic concept is to decouple the structure from the damaging effect of horizontal shaking during earthquake by allowing sliding of super structure at plinth level. This provides a low cost solution to earthquake protection employing without conventional techniques of strengthening the structural member. The present study explores three primary issues of the pure-friction isolation problem, namely, feasibility study of suitable P-F sliding interface material, analytical modelling of P-F isolation system and the shake table testing of the P-F isolated model to validate the analytical model. A summary of these studies and observations are as follows: Five different interfaces, namely, Green marble/Green marble; Green marble/High Density Poly Ethylene (HDPE); Green marble/Geosynthetic; Green marble/ rubber sheet layers and Green marble/Concrete, have been studied. The friction characteristics of these interfaces have been investigated through experiments. Two types of experimental tests have been carried out to measure the interface properties of materials, namely, shear box test and wall test.... The shear box test and wall test, show that the coefficient of friction values of Green Marble/Green Marble, Green Marble/HDPE and Green Marble/Geosynthetic lie in the desirable range, i.e., 0.05 to 0.15. These interfaces are better alternatives for sliding surfaces on account of their durability and easy availability. The masonry building with sliding interface is idealized as a two degree of freedom system force at sliding interface for rigid plastic non linear behavior of sliding structure. The average values of friction coefficients obtained from experimental tests are used in the analytical model. The sensitivity of P-F system to excitation frequency is investigated with the help of sinusoidal excitation of varying frequency. Since the nature of response (periodic, or non-periodic) has an important bearing on the performance of P-F isolation system, the effect of excitation band width on the response characteristics is also studied. A set of earthquake ground motion (artificial as well as recorded) are considered for investigating effectiveness of different P-F isolators. The spectral acceleration is found to decrease as the mass ratio increases. The spectral displacements at the base are not significantly affected by the variation of mass ratio. The sliding structure is quite effective in reducing the seismic response ofthe structure when excited by both horizontal and vertical motion. With the vertical acceleration component (in addition to horizontal component) there is no significant difference in observed peak absolute acceleration while base sliding displacement is more as compared to the case without vertical component. Hence base sliding displacement may be underestimated if vertical motion is not considered in addition to horizontal component. The frequency response (FR) function indicates that slippage prevails if the excitation frequency lies in a suitable frequency range. This range increases with higher mass ratio. For mass ratio of 1 and in the approximate FR range of 0.3 to 1.5 there is a reduction in maximum top mass acceleration for sliding interface model vis-a-vis the fixed base model. The P-F isolation system is found to be effective in the case of broad-band excitations only and that too in the acceleration sensitive range of periods. The P-F system is not effective for protection against narrow band motions for which the system response is quasi-periodic. Shake table tests are performed on V2 scaled single storf-ybase isolated brick masonry model supported by different sliding interfaces subjected to an artificial accelerogram that is compatible with design spectrum of Indian Standard (IS 1893 (Part 1): 2002) IV corresponding to the level of maximum considered earthquake in the most severe seismic zone (PGA=0.36g) in horizontal and 2/3rd of this acceleration in vertical direction. The reduction in absolute response acceleration at roof level for the isolated structure is obtained experimentally as compared to analyticallyfixed base structure which is 76%,70%, 65% and 47% respectively for sliding interfaces namely Green marble/HDPE, Green marble/Green marble, Green marble/Geosynthetic and Green marble/Rubber respectively. The difference in analytical response reduction and experimental response reduction is within 10%. It is observed experimentally that the peak relative sliding displacement is maximum for sliding interface Green marble/HDPE and the value is 94 mm while it is 50 mm, 25 mm and 2.5 mm in case of Green marble/ Green marble, Green marble/Geosynthetic and Green marble/Rubber interfaces respectively. This suggests relative sliding displacement decreases with increase in friction coefficient. The experimental peak sliding displacement for Green marble/Green marble (50 mm), Green marble/Geosynthetic (25mm) abs well within plinth projection of 75mm (3in) and can be used as a low cost distributed P-F base isolation for masonry buildings.